docs/install.md: update Fedora badge for Fedora 35

[fangyi-zhou]

Issue Description

Update OCaml version badge for Fedora.

Changes Made

Version badge used in the master bracnh is for Fedora 32 (reached EOL in May), this commit changes the badge to Fedora 35 (expected to be released in within this month).

• [x] ❗ If the PR changes a markdown document in site/, a comment was added in https://github.com/ocaml/ood/issues/52 with a link to the PR
• [x] PR is descriptively titled and links the original issue above
• [ ] Before/after screenshots (if this is a layout change)
• [ ] Details of which platforms the change was tested on (if this is a browser-specific change)
• [x] Context for what motivated the change (if this is a change to some content)

[avsm]

thanks!